On Wed, 23 Sep 2020 17:18:52 +0200
Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
> When doing an atomic modeset with ALLOW_MODESET drivers are allowed to
> pull in arbitrary other resources, including CRTCs (e.g. when
> reconfiguring global resources).
>
> But in nonblocking mode userspace has then no idea this happened,
> which can lead to spurious EBUSY calls, both:
> - when that other CRTC is currently busy doing a page_flip the
> ALLOW_MODESET commit can fail with an EBUSY
> - on the other CRTC a normal atomic flip can fail with EBUSY because
> of the additional commit inserted by the kernel without userspace's
> knowledge
>
> For blocking commits this isn't a problem, because everyone else will
> just block until all the CRTC are reconfigured. Only thing userspace
> can notice is the dropped frames without any reason for why frames got
> dropped.
>
> Consensus is that we need new uapi to handle this properly, but no one
> has any idea what exactly the new uapi should look like. Since this
> has been shipping for years already compositors need to deal no matter
> what, so as a first step just try to enforce this across drivers
> better with some checks.
>
> v2: Add comments and a WARN_ON to enforce this only when allowed - we
> don't want to silently convert page flips into blocking plane updates
> just because the driver is buggy.
>
> v3: Fix inverted WARN_ON (Pekka).
>
> v4: Drop the uapi changes, only add a WARN_ON for now to enforce some
> rules for drivers.
>
> v5: Make the WARNING more informative (Daniel)
>
> v6: Add unconditional debug output for compositor hackers to figure
> out what's going on when they get an EBUSY (Daniel)
... gmail workaround ...
> ---
> drivers/gpu/drm/drm_atomic.c | 29 +++++++++++++++++++++++++++++
> 1 file changed, 29 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> index 58527f151984..f1a912e80846 100644
> --- a/drivers/gpu/drm/drm_atomic.c
> +++ b/drivers/gpu/drm/drm_atomic.c
> @@ -281,6 +281,10 @@ EXPORT_SYMBOL(__drm_atomic_state_free);
> * needed. It will also grab the relevant CRTC lock to make sure that the state
> * is consistent.
> *
> + * WARNING: Drivers may only add new CRTC states to a @state if
> + * drm_atomic_state.allow_modeset is set, or if it's a driver-internal commit
> + * not created by userspace through an IOCTL call.
> + *
> * Returns:
> *
> * Either the allocated state or the error code encoded into the pointer. When
> @@ -1262,10 +1266,15 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
> struct drm_crtc_state *new_crtc_state;
> struct drm_connector *conn;
> struct drm_connector_state *conn_state;
> + unsigned requested_crtc = 0;
> + unsigned affected_crtc = 0;
> int i, ret = 0;
>
> DRM_DEBUG_ATOMIC("checking %p\n", state);
>
> + for_each_new_crtc_in_state(state, crtc, old_crtc_state, i)
> + requested_crtc |= drm_crtc_mask(crtc);
Is "old crtc state" the state that userspace is requesting as the new
state?
> +
> for_each_oldnew_plane_in_state(state, plane, old_plane_state, new_plane_state, i) {
> ret = drm_atomic_plane_check(old_plane_state, new_plane_state);
> if (ret) {
> @@ -1313,6 +1322,26 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
> }
> }
>
> + for_each_new_crtc_in_state(state, crtc, old_crtc_state, i)
> + affected_crtc |= drm_crtc_mask(crtc);
And after driver check processing, the "old crtc state" has been
modified by the driver to add anything it will necessarily need like
other CRTCs?
What is "new state" then?
> +
> + /*
> + * For commits that allow modesets drivers can add other CRTCs to the
> + * atomic commit, e.g. when they need to reallocate global resources.
> + * This can cause spurious EBUSY, which robs compositors of a very
> + * effective sanity check for their drawing loop. Therefor only allow
> + * drivers to add unrelated CRTC states for modeset commits.
> + *
> + * FIXME: Should add affected_crtc mask to the ATOMIC IOCTL as an output
> + * so compositors know what's going on.
> + */
> + if (affected_crtc != requested_crtc) {
> + DRM_DEBUG_ATOMIC("driver added CRTC to commit: requested 0x%x, affected 0x%0x\n",
> + requested_crtc, affected_crtc);
> + WARN(!state->allow_modeset, "adding CRTC not allowed without modesets: requested 0x%x, affected 0x%0x\n",
> + requested_crtc, affected_crtc);
> + }
This hunk looks good to me.
Thanks,
pq
> +
> return 0;
> }
> EXPORT_SYMBOL(drm_atomic_check_only);
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